osal_none: avoid ++/-- on volatile semaphore count

Increment and decrement of a volatile-qualified object are deprecated
since C++20, so including tusb.h from a C++20 (or later) translation unit
with OPT_OS_NONE warns with -Wvolatile and fails under -Werror.

Use an explicit read-modify-write instead. The generated code is the
same: count++/count-- on a volatile was already a non-atomic RMW.

Fixes #4103